The present invention relates to image forming apparatuses installed in electrophotographic printers, including electrophotographic copying machines.
Recently, there have been two rather conflicting requests from the users of electrophotographic printers whose light sources are either lasers or light-emitting diodes (LED). One is for high quality functions such as double-sided printing, composite copying on one side of a paper, high speed printing, multi-colored printing, and sorting. Another is for compactness, less weight, and inexpensiveness. These are usually considered conflicting since compactness leads to slower printing speed and to fewer functions. In fact, no option unit to upgrade compact electrophotographic printers by adding functions such as double-sided printing and/or composite copying on one side of a paper with different originals is provided.
It has been necessary, therefore, for users to purchase high quality electrophotographic printers besides the compact ones if high quality printing is required. There is indeed a device which comprises two photocopying machines juxtaposed in the paper feeding direction so that papers may flow through the copying machines continuously for double-sided copying or composite copying on one side of a paper with different originals (such as two-colored printing) as disclosed in Japanese Patent Laid Open No. 59-10958 and No. 60-195576, respectively. Each function, however, is limited to double-sided copying or to composite copying on one side of a paper respectively, and also, because the device is no more than two conventional photocopying machine connected, it requires floor space as large as that used for two copying machines.
The present invention, however, provides compact image forming apparatuses that are inexpensive and have several sophisticated functions.